At first I wanted to mod this... but I just had to reply.
There are at least 4 GOOD reasons for a 6mo release cycle. (maybe more but these are the ones I know).
1. Build Hype. Would Slashdot and a bunch of other media sources really talk about Ubuntu if it didn't make a release every 6 months.
2. Developer Fatigue. Having a 6 month release cycle allows developers to be creative and add new features for a few months then shift to bug fixing mode for a couple of months. This prevents developers from getting too wrapped up in adding new features or from getting bored just fixing bugs. IIRC many developers left Debian due to agonizingly long cycle from unstable to stable... they had to add features to keep up with other distros, but that only prolonged the bug-fix mode that they were in.
3. Support Cycles. There is a for-profit aspect to Ubuntu that many people forget about, and in order to exploit that they must put limits upon what they are willing to support. With a regular, frequent, release cycle they can be sure that their releases are timely enough to support the customers needs and they can retire support for aged releases at a specific time as well.
4. Stability. If I install KK then I know that any package upgrades (short of upgrading to a new version) are only supposed to fix things that are broken in my current install. If there were a perpetual upgrade cycle, then I would be pulling upgrades to packages that include new features, depreciate features, or even remove entire depreciated packages. This may make my system unusable for it's intended purpose.
A prolonged, or worse perpetual, cycle is nearly impossible to support, has no guarantee of package stability, is boring to many/most developers, and generates little to no press. NONE of these thing would allow anyone to profit from the development of Ubuntu, except perhaps Apple and Microsoft.